1. Field of the Invention
The invention pertains to a spectrophotometric reflectance method and apparatus for monitoring metabolism and is illustrated as monitoring cellular oxidative metabolism by conducting non-invasive, in vivo, harmless, continuous in situ measurements of changes in the steady state oxidation-reduction of cellular cytochromes together with changes in skin and bone blood volume, in organ blood volume, the oxygenation state of hemoglobin and the rate of blood flow in a vital organ such as the brain, heart or kidney, in other organs, in limbs or other parts of a living human or animal body.
2. History of the Prior Art
An extensive and useful history of the prior art is given in the referred-to copending application, Ser. No. 810,777 and which is deemed incorporated herein by reference to avoid repetition of the same.
As was pointed out in such description of the prior art, while circulatory-respiratory functions, arterial blood oxygenation and blood samples, per se, have been monitored by spectrophotometric techniques, presently existing methods are not suited for assessing the sufficiency of oxygen and metabolism in general in such vital organs as the brain and heart. Further, such prior methods do not provide precise information and are often traumatic as well. Consequently, an obvious need exists for a method by which this life sustaining parameter, i.e., cellular oxidative metabolism, can be measured in vivo, in situ and monitored continuously with precision and in a non-invasive, non-traumatic manner. Equally important is a need to be able to monitor blood volume and blood flow rate of the organ being monitored.
In order to distinguish and understand the invention of the present application, a summary of the invention of the copending application Ser. No. 810,777 is restated here for reference as what is regarded as the most pertinent prior art.
It is known that the cellular enzyme cytochrome a, a.sub.3 (also known as cytochrome c oxidase) has a key role in oxidative metabolism. That is, it has been established that the enzyme interacts directly with oxygen and mediates the release of energy during the reduction of O.sub.2 to H.sub.2 O. This is achieved by the catalytic donation of four electrons to O.sub.2 and subsequent combination with four H+ ions. Under conditions of an inadequate O.sub.2 supply, electrons accumulate and the enzyme population shifts to a more reduced steady state. Consequently, an ability to continuously measure and monitor the redox state of this oxygen utilizing enzyme in vivo, in situ would provide decisive information on the parameter of oxygen sufficiency in any tissue or organ in question. The invention of the copending application Ser. No. 810,777 provided that capability as well as the capability to monitor blood volume and blood flow rate in a manner which is non-invasive and atraumatic.
In the invention of the copending application Ser. No. 810,777, this was accomplished by optical techniques, the application of which was made possible by observing that the body and its organs are relatively pervious to low level, non-hazardous light energy in the near infrared region of the spectrum. Of particular importance, it was discovered that a beam of relatively low level, non-intense radiation in reference and measuring wavelengths of from about 700-1300 nm can penetrate, reach the organ and be detected and monitored at the end of a relatively long optical path in any selected portion of a human or animal body, which path includes bone as well as soft tissue. While such operating principles were illustrated in a manner indicating them to be useful in either a transillumination or a reflectance technique, emphasis was given in the invention of copending application Ser. No. 810,777 to a transillumination technique whereas in the present invention and application emphasis is given to use of the same basic principles in a reflectance technique.
By fortunate coincidence, cytochrome a, a.sub.3 has radiation, absorption properties in the aforenoted spectral region, the character of which varies according to its oxidation state. Thus, the invention of the copending application recognized that it is possible to monitor the redox state of this oxygen utilizing enzyme by a spectrophotometric method not known to the art prior to the invention of the copending application.
The spectrophotometric measurements, according to the invention of the copending application were made in vivo by transmitting near infrared radiation in at least two different and periodically recurring wavelengths to the test organ, in situ, and detecting and measuring the radiation intensity which emerged for assessment of biochemical reactions utilizing the Beer-Lambert Law as referred to in the copending application. One of the wavelengths selected was in a range at which oxidized cytochrome a, a.sub.3 is highly absorptive. One or two additional wavelengths outside the peak of the cytochrome absorption band, but preferably in relatively close proximity to the measuring wavelength were presented in sequence to provide one or more reference signals. A simple subtraction or ratio calculation between the measuring and reference signals was achieved by appropriate circuitry and the non-specific changes in the intensity of transmitted radiation not attributable to absorption by cytochrome a, a.sub.3 were eliminated.
Although the capability for continuously monitoring cellular oxidative metabolism by monitoring the redox state of cytochrome a, a.sub.3 in the cells of the selected organ was of principal interest to the invention of the copending application Ser. No. 810,777, ancillary data on circulatory parameters related to functioning of the organ was also shown to be obtainable. As an example, it was shown that the oxygenation state of the blood supplied to a given organ can be monitored by the hemoglobin band at slightly different wavelengths, e.g., 740-780 nm, in the aforenoted near infrared region of the spectrum. Likewise, data on the total blood volume of the organ was shown to be obtainable by monitoring a hemoglobin (Hb) oxyhemoglobin (HbO.sub.2) isobestic point. This well-known spectrophotometric term refers to a wavelength at which two forms of the same molecule or mixture of molecules have equal absorption intensity. Thus, for oxygenated and disoxygenated hemoglobin, such a point was found to occur variously between 810 and 820 nm. This variation of stated wavelengths derives from problems arising from the very low optical densities of Hb and HbO.sub.2 in this range and the relative insensitivity of most commonly available spectrophotometers in this wavelength range. In practice, any wavelength in the entire range of 815.+-.5 nm was stated to be useful without jeopardy to the results in situations where the measurements are less sensitive to small errors. As further pointed out in the copending application, a yet wider range of wavelengths can serve the purpose since even small blood volume changes will outweigh the possible interference by Hb.revreaction.HbO.sub.2 shifts. In another approach described in the copending application, the less practiced technique of combining two wavelengths with opposite optical density (OD) responses to the interfering reaction can be combined. Thus, for Hb.revreaction.HbO.sub.2 equal .DELTA.OD values but of opposite sign were shown to occur at 786 and 870 nm. This combination of signals of equal strength but opposite sign at two wavelengths is called a "contrabestic pair". It is especially useful when two reference wavelengths are used straddling the peak to be measured in conditions of intense and changing, wavelength dependent scattering. A series of wavelengths chosen such that the net sum of their optical density changes becomes zero is another method of practicing the cancellation of interfering reactions. In contradistinction, "equibestic" pairs can be used to correct for errors arising when the spectral effects of a Hb to HbO.sub.2 shift or the reverse predominate. In the case of the invention of the copending application, a reference wavelength was selected which has an equal OD effect in the same direction as the one occurring at the measuring wavelength when the interfering reaction proceeds.
In addition, blood flow rates in the invention of the copending application were monitored, albeit discontinuously, by the rapid administration of a small quantity of a dye, e.g., "cardiogreen", having absorption properties in the near infrared spectral region or alternatively by having the test subject take single breaths of a gas mixture containing a high and low concentration of oxygen in alternating sequence or one breath of a mixture with a small, innocuous admixture of CO. By selecting two wavelengths for differentially measuring the optical density of the organ in the spectral region of the absorption band of the dye, an optical signal indicating the arrival and subsequent departure of the dye in the cerebral circulation and dilution in the total blood volume, the so-called transit time, was measured. The latter was stated in the copending application to be directly indicative of the rate of blood flow as proven by Zierler (see the book "PRINCIPLES OF APPLIED BIOMEDICAL INSTRUMENTATION"). Similarly, in the invention of the copending application, the optical density differences of the hemoglobin compounds (HbO.sub.2, HbCO or other) were described as useful to provide the optical signal when the inspired air is suddenly and briefly varied. Having restated a summary of the invention of copending application Ser. No. 810,777, a summary of the present invention will be given.